1. Field of the Invention
This invention relates to crossbows, and particularly to trigger mechanisms in crossbows.
The primary objective in a crossbow trigger mechanism is to enable the drawn bowstring to be retained with high tension for maximum propulsive force, while at the same time providing a reliably smooth release with relatively low and predictable trigger pressure. A good trigger mechanism obviously results in greater accuracy being attainable, since a smooth and timely release can be more easily achieved.
Most crossbow trigger mechanisms have the same essential structure, namely a rotatable bowstring-retaining member or "catch" and a trigger which when pulled disengages the catch. Sear surfaces on the catch and trigger engage each other until the trigger is pulled. When the trigger is pulled, the sear surfaces are disengaged, and the catch then rotates under the force of the bowstring tension to release the bowstring and thus project the arrow or other projectile.
In crossbow trigger mechanisms, trigger pressure depends on the mechanical advantage of the trigger itself, and on the force of friction between the sear surfaces of the trigger and catch, since these surfaces slide across each other when the trigger is pulled. The friction between the sear surfaces is a function of the force with which the catch bears against the trigger. Since that force depends on the bowstring draw force and on the mechanical advantage of the catch, it is advantageous to have a large mechanical advantage in the catch, in order to reduce the friction. A typical bowstring draw force, and hence the force acting on the bowstring-retaining portion of the catch, is in the order of 150 pounds.
Friction can of course also be reduced by lubrication and by polishing of the sear surfaces, but such measures alone do not adequately reduce trigger pressure.
Ideally, a crossbow trigger mechanism should be able to reduce the effect of friction to the point where trigger pressure is so low that it becomes necessary or desireable to add means such as a spring to augment the trigger pressure. Spring force is consistent and reliable, producing a more consistent and reliable release point than when friction is the sole or primary variable.
The trigger mechanism should also be relatively compact, so as to avoid the need for a large stock in which the mechanism must be mounted, and so as to permit the trigger to be positioned relatively close to the plane of the bowstring and hence the line of the arrow. It is difficult to construct a mechanism which has a large mechanical advantage in the catch without having an unduly large mechanism case. The mechanism should not only be not too large in terms of depth, but also relatively short, so that the stock need not be unduly long.
A crossbow trigger mechanism must also have a safety to prevent inadvertent release of an arrow.
2. Description of the Prior Art
Many crossbow mechanisms have been known in the past, and many have been patented. Specific examples are the trigger mechanisms shown in Canadian Pat. No. 469,802 (Diehr) and in the following U.S. Pat. Nos. 2,278,535 (Dobson), 2,500,509 (Bailey), 2,609,810 (Gruner), 2,786,461 (Pelsue), 2,842,114 (Duncan), 3,490,429 (Benedict), 3,788,299 (Mathews), 4,192,281 (King), 4,206,740 (Lydon), and 4,294,222 (Pelsue). U.S. Pat. No. 1,469,610 (Ustynik) shows a crossbow-type of trigger mechanism, used to fire a rubber band.
An examination of these patents shows that the mechanical advantage of the catch in particular is in each case limited by the geometry of the mechanism, and particularly by the relative lengths of the lever arms of the catches in relation to their pivot points. In well-known crossbow trigger mechanisms previously known and presently available on the market, trigger pressures are thus generally somewhat higher than can be considered ideal, thus making accuracy difficult to achieve.